Because of the effects of heat on pistons of engines, particularly internal combustion engines, it is necessary to produce pistons which are convex and out-of-center with respect to their external skirt surface. This out-of-center has mainly in the past had a regular mathematical pattern. For example, the movement pattern produced for a stylus positioned against a piston skirt is a sin.sup.2 x-shaped movement for one rotation of the piston. Normally the amplitude of the out-of-center, but not its mathematical shape, changes corresponding to the height of the piston. The piston material is usually an aluminum alloy. Depending on the application, it is also normal to have an integral ring support cast into the piston in the ring area.
The surfaces of the piston skirt are normally machined on turning machines (lathes). The final machining of the piston skirt then takes place on a further machine whereby the out-of-center and convex shape is then formed. The depth of cut amounts to a few 1/10 mm. The cutting materials used in the aluminum range are PKD (polycrystalline diamond) and HM (hard metal) in the ring support area.
The machining of the surfaces of piston skirts is performed by NC machines with an additional mechanical copying attachment. In this case the cam rotates synchronously with the workpiece, with the cam geometry being transferred to the workpiece by a cutting tool secured to a tracer arm. The rotational speeds in this case are usually up to approximately 1500 o.p.m.
The most recent developments are concerned with trying to completely get rid of the cam and optimize the cutting speeds.
For example, the OKUMA Company has proposed a linear ancillary axis with a linear motor drive.
A similar proposal is known to have come from the ExCell-O Company.
The Cross Company has made a similar proposal, see U.S. Pat. No. 4,653,360.
Mahle GmbH has also proposed a linear ancillary axis driven by a moving coil, see German DE-PS 27 24 808.
The journal Werkstatt und Betrieb (123/1990), 5, p. 396-400 contains a description of how asymmetrical workpieces can be turned using a quick-response servosystem.